Facebook pixel doesn’t always work, even when installed and set up correctly. This is due to the technical details.
Marketers don’t speak about that because, I believe, they’re unaware about those technical details. Assuming that you install a pixel on your website and thus you’ll be able to track each person who is on your website or has visited it, is naive.
In what cases does Facebook pixel not work?
When a visitor has no Facebook account
When a visitor does have a Facebook account, but isn’t currently logged in into it.
When a visitor opens his Facebook only in containers. In Firefox there’s such a possibility. I do this. Namely, when if I’m logged in into my Facebook account, and I visit a website that my have a Facebook pixel at it, a pixel won’t work because my Facebook lives in a different container.